The sound produced by the electric guitar is strongly dependent on the guitar amplifier. All guitar amplifiers contain a preamplifier, which may have a frequency dependent input stage (for example the ‘bright’ input on some amplifiers), one or more distortion stages, and a tone control stage. The preamplifier output is fed to the power amplifier section. The power amplifier generates the signal required to drive the speaker, and may also operate in a nonlinear fashion at high levels. The amplifier output is fed to a loudspeaker, which converts the amplified signal into an acoustic signal. The loudspeaker may also operate in a nonlinear fashion.
The guitar amplifier thus has the role of altering the guitar signal by linear filtering, and nonlinear distortion, which is the reason why it is so critical to the guitar sound. The nature of the nonlinear mechanisms in the amplifier and their relationship to the linear elements are what determines the sounds that are recognised as being associated with each musical genre (rock, blues, etc).
The guitar amplifier has been in existence for over forty years, and the range of products available today is more diversified than it has ever been. Both solid state and valve (tube) amplifiers are available, both in combo and discrete form. In addition, a plethora of discrete guitar preamplifiers are available which seek to mimic the sound produced by the guitar amplifier at controllable volumes, and which can be input to mixing desks for recording purposes, or which are input directly to power amplifiers with either hi fidelity PA speakers, or guitar speaker cabinets.
In spite of the years of development since the early amplifiers, the original technology still maintains a pre-eminent position in the marketplace. The valve amplifier is still preferred by many musicians, and the speakers used are practically identical to those of the original amplifiers. Early tube amplifiers were relatively simple, and had high distortion figures, and when overdriven produced a sound that was perceived as musical. When solid state (semiconductor) amplifiers appeared on the market, they were expected to be an improvement over valve technology. Both the musicians did not agree. For example, Jimi Hendrix used solid state amplifiers for a time, but returned to valve amplifiers because he preferred their sound. Valve amplifiers have since come to be regarded as better sounding, although solid state amplifiers have been further developed and now produce more reasonable imitations.
Many opinions have been expressed as to why valve amplifiers sound better than solid state amplifiers, but few of these opinions have been backed up by scientific evidence, and few of them take into account all of the characteristics of the valve amplifier that could contribute to its preferred status. Some of the characteristics that differentiate valve and solid state amplifiers, and the effect of the loudspeaker (which is common to both) are mentioned below by way of background.
Nonlinear and overload characteristics of valve and semiconductor devices: A common belief is that valves have a soft clipping characteristic which does not produce sharp discontinuities in the signal waveform, and avoids the production of high harmonics. However, it is simple to construct solid state nonlinear circuits with compression characteristics that are even softer than those of valves, but which do not produce the same results subjectively.
Another popular opinion is that valves produce even order distortion products, whereas solid state amplifiers produce odd order harmonics. However, the truth of this statement is dependent on the circuit topology and the degree of overdrive. In a preamplifier stage, the valve/transistor is operated in a class A mode, and both devices produce even and odd order distortion. However, the valve will produce a different set of harmonics to the transistor. Hamm, has demonstrated, for example, that valves have harmonic ‘signatures’ which are different to those of transistors, and in many cases the valve produces less high harmonics than the transistor (see R. O. Hamm, “Tubes versus Transistors—Is There An Audible Difference?” J. Audio Eng. Soc., vol. 21, no. 4, pp 267–273, May 1973). The harmonic signature of the valve is more closely matched by that of the field effect transistor, which has a quadratic relationship between the drain current and gate voltage, similar to the plate current verses grid voltage characteristic of a valve.
When a preamplifier valve is serverly overloaded, the gate voltage can exceed the cathode voltage. In this case conduction occurs between the gate and cathode, and the positive peaks of the applied input waveform are clipped. It is claimed that this produces a bias shift in the waveform, and that this creates a significant amount of second harmonic distortion (see European patent application 95300062.7, “Multi-stage solid state amplifier that emulates tube distortion”, Peavey Electronics Corp.). A multi-stage preamp with several valves creates a high level of distortion, clipping and sustain that is recognised as having a tube preamplifier distortion characteristic.
A number of different solid state devices have been used to attempt to create subjectively acceptable distortion. Germanium and silicon transistors were used in early fuzz box products. Diode clippers have been employed. Diodes in the feedback loop of an operational amplifier create a softer clipping characteristic because the gain of the op amp reduces as the diode starts to conduct. Light emitting diodes have also been employed in this manner. Transistors can be employed in the same way to allow voltage control of the transfer characteristic. Even CMOS inverters have been biased to create nonlinear amplifier stages.
In a power amplifier, two output power devices are typically operated in class B or AB mode. This produces a transfer characteristic which is symmetrical, and as a result, the circuit produces only odd order harmonics, whether it is a valve stage or transistor stage. However, when the power amplifier is severely overloaded, the output waveform is more dependent on the device characteristics. Grid conduction and bias shift occurs in a valve power stage, creating an output waveform which demonstrates crossover distortion, and this waveform is claimed to be subjectively superior to that of the solid state waveform (see European patent application 95300126.0, “Solid state tube compression circuit”, Peavey Electronics Corp.).
The output transformer: The operating characteristics of valves require that an output transformer be used to match them to low impedance speaker loads. However, practical transformers have leakage inductances and inter-winding capacitances which limit their performance. There are a number of consequences of this fact. Firstly, high levels of negative feedback cannot be applied to the valve amplifier, because the transformer response at high frequencies is likely to cause instability. This means that valve amplifiers have lower levels of global negative feedback than solid state amplifiers, and higher distortion figures.
A second consequence is that the transformer does not have a flat response, and therefore contributes some filtering to the distorted signal from the power amplifier stage, particularly with low negative feedback.
A third consequence is that iron cored transformers are nonlinear. The magnetizing current in the primary coil is a nonlinear function of the applied voltage, and unless the source has a low impedance, this magnetizing current produces distortion in the output waveform (see N. G. R. Partridge, “Harmonic Distortion in Audio Frequency Transformers,” Wireless Engineering, p 394, September 1942; p 451, October 1942, p 503 November 1942). This form of distortion can occur in pentode output stages due to the high output impedance of the pentode.
Power Supplies: Valve power supplies tend to have poorer regulation than solid state supplies, and the plate voltage can fall when the output sate of a valve amplifier overloads, producing a greater degree of compression of the signal.
Output impedance: The interaction between the amplifier and speaker differs between valve and solid state devices, due to their different output impedances. A low output impedance solid state amplifier is better able to control the loudspeaker motion, whereas the higher output impedance valve amplifier allows the natural response of the speaker to be more perceptible. This is most noticeable at the low frequency resonance of the speaker.
One of the more overlooked factors in the sound of the guitar amplifier is the performance of the loudspeaker, and yet it is the probably the single most important contributor to the acceptable sound produced by an overdriven amplifier. All guitar amplifiers use one or more single element loudspeakers. Typical configurations are a single 12 inch, two 12 inch (sometimes in a stereo chorus format) and four 12 inch speakers in a “quad box”. No commercially successful guitar amplifier has ever used a woofer-tweeter two-way speaker cabinet. There are good reasons for this fact.
The response of a typical 12 inch loudspeaker designed specifically for guitar amplifiers is shown in FIG. 1. The speaker was loaded in a cabinet with an open back and tested in an anechoic chamber at a distance of three meters away, using a commercial maximum length sequence measurement system and laboratory grade microphone. The response rolls off at low frequencies because the open backed cabinet allows cancellation of the front radiated sound by the back radiated sound. In the region of 1 kHz the first cone resonance occurs, and the response drops off noticeably, and then increases again to a peak at just above 2 kHz. The behaviour of the speaker when excited by a sinewave was examined using a small microphone placed close to the speaker diaphragm, in order to detect the local vibration of the speaker. As the input frequency approached 1 kHz the vibration at the periphery of the speaker, away from the voice coil, began to drop off, due to a lack of stiffness in the speaker cone. This reduced the radiated power considerably, creating the low output seen at above 1 kHz. At frequencies above this point, the speaker began to vibrate in a resonant mode with a circular antinode between the center and the rim, and the center and rim vibrations being out of phase. Since the rim vibrating area is large, the output is large in this frequency region. The radius of the antinode varied with the applied frequency.
At frequencies above 2 kHz the response drops to about 6 kHz, where it falls rapidly away. The modal behaviour of the diaphragm becomes complex at these frequencies, and little power is radiated in the far field.
Hence, the speaker behaves as a filter which rolls off the bass, puts a notch at around 1 kHz, and places a low pass filter at about 6 kHz.
The rapid attenuation of frequencies above 6 kHz is critical to the subjectively acceptable sound of the distorted guitar signal. The nonlinearities create a large amount of high frequency harmonic and intermodulation distortion, and this would be extremely unpleasant unless it was attenuated, whether using valve or solid state devices.
The notch at around 1 kHz is the other important feature of the speaker response. It removes the “mid rangy” sound of the guitar signal and creates a “thin” sound which has become recognised and preferred as the rock guitar tone.
All high quality preamplifiers that are designed to be plugged into a mixing desk or PA system contain speaker simulators which mimic the frequency responses of one or more speaker cabinets.
A further effect produced by a single loudspeaker is Doppler distortion. If a high frequency signal and a low frequency signal are radiated simultaneously, the high frequency signal is Doppler shifted by the low frequency. This effect is negligible in two or three way speakers, and some writers have claimed it is not noticeable in single speakers, but the high frequency, high amplitude distortion products produced by a guitar amplifier may be frequency modulated by the lower frequencies to the point where it affects the perceived sound.
Some amplifier manufactures make only valve products. Other manufacturers produce both valve and solid state amplifiers. The majority of amplifiers sold are solid state, because these can be made more cheaply, and with more reliable performance. Many manufacturers are increasingly looking for solid state alternatives which produce similar sound to the “valve paradigm”. For example, Peavey Electronics Corp has released a line of amplifiers termed “Transtube™” which they claim reproduces the distortion characteristics of tube amplifiers—see European patent applications 95300062.7 and 95300126.0 on the Transtube technology, covering both valve preamp and valve power amplifier behaviour. Peavey have stated that valves will ultimately be unavailable and that viable alternatives must be found.
Another company, Deja Vu systems, has produced a tube emulator which uses solid state nonlinear circuitry to emulate the nonlinearity performance of the commonly used 12AX7 dual triode tube—see E. K. Pritchard, “The Tube Sound and Tube Emulators,” dB Magazine, p 22–30, July/August 1994 and U.S. Pat. Nos. 4,869,336, 4,995,084 and 5,113,014.
An alternative approach to creating the guitar sound is to develop a preamplifier that produces the same, similar, or superior subjective response without attempting to simulate the overload characteristics of preamplifier and power amplifier valves, and to use standard amplification and speaker systems to radiate the signal. There are disadvantages and advantages in this approach.
Disadvantages include:                New distortion circuitry must be developed that is subjectively equivalent or superior to valve circuitry.        Standard PA speaker systems vary widely in their level of fidelity. Many speaker cabinets have extremely non-flat responses, and colour any sound they radiate. The consistency of the guitar sound thus cannot be guaranteed unless specific speaker systems are recommended. The advantage of the guitar speaker is that it is the final EQ stage in the signal chain, and cannot be subsequently coloured.        High fidelity two-way speakers are more expensive than the single radiator guitar speaker.        Power amplifier distortion must be avoided, since the PA speaker will not filter any distortion products it produces. Many modern amplifiers contain clipping avoidance circuitry which reduces this risk.        
Advantages are:                The artist need only buy a new preamplifier when he/she desires a new sound        The directional radiation characteristics of a high fidelity speaker are superior to a guitar speaker system. A 12 inch speaker cannot radiate high frequencies over wide angles. For example, the −3 dB beamwidth of a 12 inch speaker at 4 kHz is around 16 degrees. If the speaker cabinet is on the floor, the on axis response is only obtained when the listener's ears are near floor level. The only solutions are:                    a) the guitar speaker is raised on a stand and the guitarist stands in one playing position on-axis. This constrains the guitarist's position unduly.            b) A number of 12 inch speaker are used to create a wider near field radiation pattern. Many guitarists use a minimum of four speakers in a quad box, and most professional guitarists use a number of quad boxes. Interestingly, this technique does not in theory improve the radiation characteristics in the far field if the speakers are regularly spaced—it merely creates interference fringes (the array factor) in the original speaker radiation pattern. However, in practice the guitarist stands within the near field of the speaker array, and the radiation characteristics are an improvement over that of the single speaker.                        A high fidelity two-way speaker system uses a tweeter to radiate the high frequencies, which has a much greater beamwidth at these frequencies. In this case the high fidelity approach is cheaper than the original approach if good radiation characteristics are required.        The preamplifier can be designed with a single nonlinear section that creates a subjectively desirable result. This is a great simplification over the original technology in which there are a myriad of nonlinearities occurring throughout the signal chain, and which make the design process complex.        
One of the most important components of a guitar preamplifier is the distortion circuitry it employs. As stated above, many different devices have been used to create nonlinear transfer characteristics, but in all cases the goal is to create distortion that is not harsh, and which produce subjectively pleasing results when chords or intervals are played. These two subjective parameters are controlled by the harmonic and intermodulation products generated by the distortion circuit.
Control of harmonics: In general, any nonlinear circuit without memory has a voltage transfer characteristic vout=g(vin) that can be written as a power series
      g    ⁡          (              V        in            )        =            ∑              n        =        1            ∞        ⁢                  a        n            ⁢                        v          in                n            
If a sinusoid of frequency fo is applied to the network, each vinn term creates a harmonic nfo, plus other lower order terms. Hence the harmonic spectrum is governed by the coefficients in the power series. Transfer characteristics with sharp discontinuities will have a power series with large amplitudes of high order terms, creating considerable high frequency distortion products. Thus, the distortion properties are directly related to the smoothness of g(vin). For guitar distortion, where the goal is to increase sustain and compress the dynamic range, g(vin) has a compressive characteristic which has a large slope for vin=0 and reduces to a small value for large positive or negative values of vin. Smooth distortion characteristics are desirable for music such as rhythm and blues, whereas transfer curves with sharper transitions and flatter compression (very low derivatives for large |vin|) are more desirable for rock music where higher harmonic content is required.
Control of intermodulation: Most circuits do not provide independent control of intermodulation. There are, however, two ways of achieving this.                If the guitar has a hexaphonic pickup, each string may be distorted separately, which produces no intermodulation between strings. However, the sound is musically uninteresting, and the output level is dependent on the number of strings played. By allowing controlled mixing of the separate string signals before the distortion circuits, the intermodulation can be controlled and the level made more constant. This technique requires a special pickup, and is therefore impractical in most cases.        The second method of controlling intermodulation is to use frequency dependent band splitting. The guitar signal is split into a number of separate frequency ranges, and each band is distorted separately, then the bands are recombined. If an input signal is applied which has several frequency components, some components will fall within one filter band, and others will fall within other bands. The distortion circuits will distort the filtered signals separately, producing lower levels of intermodulation products between them. The system is particularly effective for non harmonic inputs such as chords, where the intermodulation products are not harmonics of the fundamentals. This technique was described by C. Anderton in (see C. Anderton, “Fuzz and Filter Fun,” Guitar Player Magazine p 120, June 1983), and he later published a four channel system termed the quadrafuzz (see C. Anderton, “Build the Superversatile Quadrafuzz—Four Fuzzes in One with Active EQ,” Guitar Player Magazine, p 37–46, June 1984). The sound was claimed to have some of the characteristics of valve distortion.        
However, any practical filter bank used of the band splitting operation will have finite attenuation verses frequency slopes in their out-of-band frequency regions. Hence for a sinusoidal input applied to an N-band system, all outputs will produce a finite amplitude sinusoid. This feature is in fact a desirable one, since the multiband distortion system is required to operate similarly for all input fundamental frequencies. Ideal filters would produce very different results depending on the fundamental frequencies applied.
The output of the barid splitting filters are fed to their respective distortion stages, where the signal is compressed and distorted. Since the input sinewave appears at all of the filter outputs, the distortion stages will tend to bring all of the input signals to the same amplitude at their outputs. Therefore, the waveform resulting from the addition of the outputs is strongly dependent on the phase of the input signals applied to the distortion stages. If the phases are not the same, the sum of the distorted signals will produce sharp transitions at the zero crossings of the signal. As an example, the output of a two channel system with a phase error of 30 degrees is shown in FIG. 2. The waveform resembles that due to crossover distortion, but the mechanism is different, and the subjective effect is undesirable.